The drive trains of vehicles known from common practice are usually made with internal combustion engines which run at a minimum speed. To start off when the vehicle is at rest, a speed gap between the lowest engine operating speed and the static transmission input shaft of a transmission unit has to be bridged by way of a speed converter or a starter mechanism. As is known, such starter mechanisms are made as a dry clutch in the case of manual shift transmissions and in the case of conventional automatic transmissions as a hydrodynamic torque converter or in the form of an automated starter clutch as a wet-running disk clutch.
To reduce or eliminate the hydraulic losses that occur in the area of hydrodynamic torque converters, it has become customary to associate converter bridging clutches with torque converters. In the engaged condition of a converter bridging clutch, a torque transfer in a vehicle drive train is bridged in the area of a torque converter. The torque transfer then takes place essentially with lower losses via a connection made by friction.
Here, a fundamental distinction must be made between two different system groups of starter mechanisms that comprise a torque converter and a converter bridging clutch corresponding thereto.
In a first system group the hydraulic actuation of the converter bridging clutch is integrated in the hydraulic fluid circuit of the corresponding hydraulic torque converter, such systems being known as two-line converters. A converter clutch piston of a two-line converter, provided in order to actuate the converter bridging clutch and that can be acted upon by hydraulic pressure in the area of a piston chamber, is formed as a flexible disk whose hub is in a rotationally fixed connection with a turbine of the hydrodynamic torque converter.
When the converter bridging clutch is disengaged, the converter clutch piston is acted upon by a flow of oil coming from an inlet side of the hydrodynamic torque converter, which flows through a hydraulic chamber of the starter mechanism towards a return side of the hydrodynamic torque converter and the piston chamber and flows around the piston. To close the converter bridging clutch an inlet pressure of the hydrodynamic torque converter is reduced essentially to zero or cut off so that a static total pressure existing in the hydraulic chamber of the hydrodynamic torque converter also falls essentially to zero. In the latter operating condition of the hydrodynamic torque converter the converter clutch piston, which is spring-loaded in the closing direction of the converter bridging clutch and which is coated with a friction lining on a side facing away from the hydrodynamic torque converter, is pushed by the spring mechanism of the converter clutch piston towards the housing on the pump side of the hydrodynamic torque converter.
To increase the transmission capacity of the converter bridging clutch and thus also the torque that can be transferred by the converter bridging clutch, an inlet pressure of the converter bridging clutch is increased. This increase of the inlet pressure of the converter bridging clutch increases the pressure of the converter clutch piston against the housing of the pump side of the hydrodynamic torque converter. Moreover, when the converter bridging clutch is engaged the converter clutch piston reduces the oil flow through the hydrodynamic torque converter to a predefined minimum so that the converter clutch piston of the converter bridging clutch performs essentially the same function as a conventional hydraulic one-way valve.
To avoid compromising the driving comfort, the converter bridging clutch is preferably only engaged in drive train operating conditions during which, effects acoustically perceptible by the driver in the vehicle and caused by speed irregularities of an internal combustion engine of the drive train, are absent. In opposition to this, however, is the desire to close the converter bridging clutch as soon as possible during a starting process in order to reduce the hydraulic power loss that occurs in the hydrodynamic torque converter when the converter bridging clutch is disengaged.
From Automobiltechnischen Zeitschrift (Journal of Automotive Technology) 97 (1995), No. 10, pp. 698-706, “Electrohydraulic Control and External Shifting of the WSA 330/580 Automatic Transmission by Mercedes-Benz”, a device is known for the operation of a hydrodynamic torque converter with a corresponding converter bridging clutch. The converter bridging clutch is actuated separately from the hydraulic fluid circuit of the torque converter. The piston chamber of the converter bridging clutch can be acted upon by the hydraulic actuation pressure needed for actuating the converter bridging clutch via a control line, separate from the hydraulic through-flow area of the torque converter while, as in the case of a two-line converter, the converter bridging clutch is spatially integrated in the housing of the torque converter. Such starter devices are customarily referred to as three-line converters and in that case belong mainly to the second system group.
The known devices for operating starter mechanisms of the first and those of the second system groups have control lines connected by valve mechanisms of the devices which are, in each case, connected by hydraulic lines of a hydraulic line system to the torque converter and the converter bridging clutch so as to be able to apply the respective necessary actuation pressures in a manner that depends on the operating conditions.
Similarly to two-line or three-line converters, starter mechanisms made as wet disk clutches are constructed with a so-termed clutch chamber and with a hydraulic chamber. The clutch chamber being acted on by hydraulic pressure to actuate the disk clutch while the hydraulic chamber can be acted upon by hydraulic pressure to cool and lubricate the disks engaged with one another.
Both in the starter mechanisms of the said two systems groups and also in wet-operating disk clutches in which the piston chamber and the hydraulic chamber are at least in part spatially separated from one another by the clutch piston. The hydraulic pressure present in the hydraulic chamber acts disadvantageously against the hydraulic pressure in the piston chamber during a engaging process of the disk clutch or of the converter bridging clutch associated with the torque converter.
In this, it is particularly disadvantageous that the hydraulic pressure in the hydraulic chamber can only be determined with technical difficulty so that the hydraulic pressure applied in the piston chamber to engage the disk clutch in unfavorable situations of a drive train, during which the hydraulic pressure in the hydraulic chamber becomes unacceptably high, is insufficient to produce the required operating-situation-dependent transmission capacity of the disk clutch. In particular, critical operating situations of a drive train, ignorance of the hydraulic conditions prevailing in the hydraulic chamber has the result that the transmission capacity of the disk clutch during a starting process is reduced by an increase of the hydraulic pressure in the hydraulic chamber to such an undesired extent that driving comfort is compromised by irregularities in the variation of a drive output torque applied at the output of the drive train or the vehicle.
Accordingly, the purpose of the present invention is to provide a device for the operation of a starter mechanism that can be brought into active connection with a hydraulic supply circuit of a transmission unit by way of which irregularities in the variation of a drive output torque that compromise a high level of driving comfort are reliably avoided.